130 AGRICULTURAL BACTERIOLOGY 



high in nitrogen as before the crop was grown; but this 

 can never be true of potassium, phosphorus, and sulphur. 



The legume bacteria are motile. There is, however, no 

 reason to believe that they can pass through the soil in a 

 horizontal direction for any distance. The plant root must 

 come in contact with the bacteria before infection can take 

 place. Since the plant roots do not fill all the spaces of the 

 soil, it is essential that the soil contain great numbers of 

 these organisms in order that an abundance of nodules may 

 be formed. If artificial inoculation is to be resorted to, it 

 is important to bring the organisms in intimate contact with 

 the roots of the young plant. This is best accomplished by 

 the inoculation of the seed rather than by the inoculation 

 of the soil. In the case of inoculation with soil, the seed 

 may be moistened slightly and the fine soil thoroughly 

 mixed with it. The seed should be treated shortly before 

 it is to be sown. The pure cultures are usually added to 

 water, which is sprinkled over the seed. It must be re- 

 membered that unfavorable conditions, such as drying and 

 sunlight, may destroy the organisms on the seed. If soil 

 containing the organisms is available in unlimited amounts, 

 it may be broadcasted over the field, or it may be applied 

 with a drill and well harrowed in, so as to mix it as inti- 

 mately as possible with the soil. 



No very definite direction can be given as to the amount 

 of soil that should be used in the inoculation, since this will 

 be determined by the number of bacteria in it. In case 

 it is broadcasted over the land, several hundred pounds an 

 acre should be added. It has been found in experimental 

 work in the greenhouse that the bacterial content of the 

 soil to which sugar has been added may reach a point where 

 one half pound an acre will produce a good inoculation. 



Another practical method of securing optimum conditions 

 favorable to the growth of a different kind of legume is to 



